1. Field of the Invention
The invention deals generally with analog to digital data conversion and, in particular, with a method and apparatus for combining a flash analog to digital converter with digital to analog functions.
2. Related Art
Systems which process signals using digital signal processing techniques require that analog signals be converted to digital form. Such conversions have been accomplished using analog to digital converters. Techniques employed in analog to digital converters have included successive approximation and subranging techniques. Time delays inherent in apparatus implementing such techniques have given rise to the development of the flash analog to digital converter.
A diagram of a relatively simple 2-bit flash analog to digital converter is shown in FIG. 1. An analog input signal, which is to be converted to digital form, is applied at Vin and transmitted over input channel 101 to a first input of comparators 102-105. A voltage reference Vref is applied across a voltage divider network 106, shown in FIG. 1 as having resistors 107-111. Voltages Vref1-Vref4 are developed at the junctions of the resistors and are applied to second inputs of comparators 102-105. The output of each comparator is a logical 1 or logical 0 depending on whether the voltage applied at the first input exceeds the reference voltage applied to that comparator. The logical ones and zeros output by comparators 102-105 are then applied to a digital output encoder 112 to produce a 2-bit digital output on signal lines 113 and 114.
As illustrated in FIG. 1, a 2-bit flash analog to digital converter has five resistors in the voltage divider network 106. A flash analog to digital converter of necessity uses a large resistive voltage divider to generate multiple reference voltages to compare against the input during the conversion process. As the number of output bits increases, the number of resistors in the voltage divider also increases. Typically, the resistive voltage divider is the largest and hence, most expensive silicon structure of the converter.
Digital to analog converters have been implemented with resistive divider networks and switches which switch in various points on a ladder network in response to digital codes. Thus resistive voltage dividers find use in both analog to digital and digital to analog converters.
Systems requiring both analog to digital and digital to analog conversion simultaneously, often use separate devices for such conversions. Such separate devices contribute to systems costs and may introduce performance compromises as a result of variations in the corresponding performance of the analog to digital converter and the digital to analog converter. In particular, voltage references used in the converters may be slightly different due to variations in the components of the separate devices, thereby introducing errors between respective digital to analog and analog to digital conversions.